This year we have continued our studies of the molecular basis of drug-induced hepatitis (DIH), a rare but often life-threatening toxicity that is a major reason for clinical trials of drugs being stopped and drugs being withdrawn from clinical use post-marketing. Although the mechanisms of DIH have not been clearly defined, due in large part to a lack of animal models, we believe that various factors besides the formation of toxic reactive metabolites may have a role in determining the susceptibility of an individual to DIH. We have explored this possibility with the use of transgenic mice and gene chip expression array technology and have discovered several anti-inflammatory factors that might predispose an individual to DIH. In one study, it was found that animals deficient in cyclooxyenase-2 (COX-2) were more susceptible to acetaminophen (APAP)-induced liver injury and death than COX-1 deficient and wild type mice. This susceptibility appeared to be related at least in part to a defect in the production of several heat shock stress proteins, detected by gene chip expression analysis. The findings are important because they suggest that individuals deficient in COX-2 express in the liver or those being treated with selective inhibitors of COX-2 may be more sensitize to DIH. In this regard, it was also found that celecoxib, a selective COX-2 inhibitor used to treat osteoarthritis, also sensitized mice to the hepatotoxic effects of APAP. In another study, APAP treatment of mice caused a significant increase in serum levels of interleukins (IL)-4, 10 and 13, cytokines that regulate inflammatory mediator production and cell-mediated autoimmunity. When mice deficient in IL-10 were treated with APAP, most of the animals died of liver injury within 24 to 48 hours. This increased susceptibility to APAP-induced liver injury appeared to correlate with an elevated expression of liver pro-inflammatory cytokines, tumor necrosis factor-alpha and IL-1, as well as inducible nitric oxide synthase (iNOS), which can lead to the formation of toxic nitrogen metabolites. In this regard, mice lacking both IL-10 and iNOS genes were protected from APAP-induced liver injury and lethality when compared to IL-10 deficient mice. All strains, including wild type animals, generated similar amounts of liver APAP-protein adducts indicating that the increased susceptibility of IL-10 deficient mice to APAP hepatotoxicity was not due to an enhanced formation of APAP-protein adducts. These findings suggest that an important feature of the normal response to drug-induced liver injury may be the increased expression of anti-inflammatory factors such as certain COX-2 products, IL-10, and others. Perhaps certain promoter region polymorphisms of these factors may have a role in determining the susceptibility of individuals to DIH.